138 COPE. (Von. Ti, 
“1. That the mechanical force used in locomotion during the 
struggle for existence has determined the digits which are now 
performing the pedal function in such groups as have undergone 
digital reduction. 2. That where the distribution of mechanical 
strains has been alike upon all the digits of the manus or pes, 
or both, they have remained in a state of approximate uniformity 
of development.” In the same year, in discussing the origin of 
the great development of the incisor teeth in the Rodentia,} 
Professor Ryder, in summing up, ventured “the reflection that 
the more severe strains to which they were subjected by enforced 
or intelligently assumed changes of habit, were the initiatory 
agents in causing them to assume their present forms, such 
forms as were best adapted to resist the greatest strains without 
breaking.” In 1878 the writer? advanced the following propo- 
sition: ‘Change of structure is seen to take place in accordance 
with the mechanical effect of three kinds of motion, viz., by 
friction, pressure, and strain.” Inthe same year Professor Ryder 
went into a discussion of the specific application of strains in 
the evolution of the dental types of the Diplarthrous Ungulata, 
and prepared the field for work in the Rodentia and Proboscidia. 
In 1879 the writer gave mechanical reasons for the reduction of 
the sectorial teeth of Carnivora to one, and for its present posi- 
tion in the jaws. In 1881 the writer® described the specific 
action of impacts and strains in the production of the existing 
characters of the articulations of the limbs in the higher Mam- 
malia. In 1887 the same subject, together with that of the 
mechanical origin of the characters of the molar teeth, was more 
fully investigated in a paper on the Perissodactyla.6 In 1888 
the writer published a paper on the mechanical origin of the 
sectorial teeth of the Carnivora,’ one on the mechanical origin 
of the peculiar dentition of the Rodentia,’ and a third on the 
mechanical origin of the dentition of the Amblypoda.? In 1889 
1 Proceedings Philadelphia Academy, 1877, p. 318. 
2 American Naturalist, 1878, January; Origin of the Fittest, p. 354. 
3 Proceedings Philadelphia Academy, 1878, p. 45. 
4 American Naturalist, March, 1879. 
5 American Naturalist, April and June, 1881. 
6 American Naturalist, 1887, pp. 985, 1060. ; 
7 Read before the American Association for the Advancement of Science, New 
York, 1887, p. 254. 8 American Naturalist, January, p. 3. 
9 Proceedings of the American Philosophical Society, 1888, p. 80. 
